The present invention is a method for the on-line, nondestructive determination of the internal bond of composite panel products. The method is particularly useful as a process and quality control tool wherein it can give an immediate appraisal of the internal bond strength of products such as wood fiber, particle, flake or strand boards. Measurements made shortly after pressing products of this type correlate well with conventional laboratory methods made several hours or days later.
For composite wood products such as those named above, internal bond strength and flexural strength are probably the two most important physical indicators of board quality. Flexural strength, sometimes termed modulus of rupture, is most indicative of surface strength while internal bond is more indicative of the interior or core strength of the product. In the past, it has been a matter of hours, or even days, before the results of quality control tests would be available to production personnel. This often resulted in large quantities of products being made which were outside quality specifications with nobody being aware of the problems.
In recent years equipment has been developed to attempt to non-destructively measure certain physical properties either on line during production or shortly thereafter. For example, Collins, in U.S. Pat. Nos. 3,423,991 and 3,780,570, shows systems using rolling transducers useful for determining voids or delamination in plywood. One transducer directs a pulse of ultrasonic energy against one face of the panel. An opposing transducer analyzes exponentially decaying reverberations to obtain the desired information. Other devices measure pulse propagation velocity as an indicator of the physical property being measured. One such device is shown in U.S. Pat. No. 4,073,007 to Boivin. Here the time for a pulse to travel a predetermined distance is used as an indicator for measuring lack of planarity of a tensioned sheet metal strip. Logan, in U.S. Pat. No. 4,201,093, uses a rolling oil filled transducer to introduce ultrasonic signals into sheets.
A common feature of the Collins and Logan devices is the use of a transducer element contained within a rolling wheel or cylinder. Various means are used to couple energy from the transducer to the wheel and from there into the material being measured.
Devices of the types just described have achieved only limited use for a number of reasons. In many cases the reliability is very poor. In other cases, the device is unable to measure the desired physical property with sufficient accuracy and/or reliability. A system described in U.S. Pat. No. 4,036,057 to Morais has been used to determine adhesion or delamination in wood products and the tensile strength of graphite-epoxy composites. This uses an ultrasonic signal to propagate a stress wave through the material being tested. A receiving transducer detects the "ringdown" or decay of resonant waves produced within the substrate being tested. A device of the Morais type has been shown to give results having a general correlation with the internal bond strength of composite panel products such as particleboard. However, this correlation is so poor that is not usefully predictive for use as a quality control tool.
The present invention has overcome the problems experienced with prior art on-line testing devices and is capable of the accurate determination of the internal bond strength of numerous types of composite panel products.